Solvent dewaxing of waxy hydrocarbon oils involves the direct or indirect chilling of the solvent/oil mixture to a temperature low enough to precipitate out the wax in the oil. The precipitated wax is separated from the solvent/oil mix by filtration using rotary vacuum or pressure filters. The filter medium used is a natural or synthetic textile cloth (usually non-woven) which is held in place on the outer surface of the filter drum by means of caulking bars and tightly wrapped with wire. The normal life expectancy of the filter cloth ranges from one to four years, after which the well worn cloth is replaced in its entirety.
Filter cloths are also susceptible to damage which is evidenced by a loss in retentive ability indicated by the presence of wax in the filtrate. Such damage takes the form of holes, rips, tears, cuts, gaps etc. and can be caused by any number of things. The wax removal doctor blade in the filter can snag the surface of the cloth, improper installation, minor unobserved defects in the cloth, etc. can lead to localized damage and failure of the cloth.
Such localized damage as compared to a general wearing out of the cloth, is treated at the site of the damage rather than by total replacement of the cloth.
The currently practiced site specific filter cloth repair technique involves blanking off the damaged cloth section by sliding a thin metal shim or plate under the hold down wires on the filter drum and onto the face of the filter cloth over the area of damage. This method does not always prove satisfactory because there is no true seal between the cloth and the overlying metal patch and the metal patch cannot be effectively used if the damage is near the edges of the drum or adjacent the caulking bars in the drum.